1. Field of the Invention
The present invention relates to a multi-brush of a rotary electric machine and a method of manufacturing the brush.
2. Description of the Related Art
JP-A-9-49478 or its corresponding U.S. Pat. No. 5,712,522 discloses a brush formed of a thick high-conduction member and a thin low-conduction member, which are bonded together.
Although there is no showing in the above publication how to connect the pigtail to the brush, this is very important to the performance of the rotary electric machine. If the pigtail is connected to the high-conduction member of the brush element, the connection resistance is low so that a good connection characteristic can be provided. On the other hand, if the pigtail is connected to the low-conduction member, the connection resistance is high, resulting in a bad connection characteristic. Because the pigtail that is inserted into the brush element from the low-conduction-member-side contacts the low-conduction member, the connection resistance becomes high, and the rotary electric machine having such a brush operates at a low efficiency. In addition, joule heat of the connection resistance may cause overheating problems.
In order to prevent the above stated problems, there is an idea that the pigtail is inserted into a deep hole of the brush element extending to the high-conduction member. However, when the pigtail is inserted in the deep hole, the low-conduction member may stick to the outer periphery of the pigtail. This increases the connection resistance, resulting in the above-stated problems. In addition, a variety of the sticking conditions of the low-conduction member to the pigtail causes a variety of the connection resistances, resulting in unstable operation. It is sometime inevitable that the pigtail is inserted to the brush element from the low-conduction member.
In order to prevent the connection resistance from increasing too much, a brush and a method of manufacturing such a brush are proposed in JP-A-2-86081. The pigtail is covered with a columnar (cylindrical or rectangular) high-conduction cover so that the pigtail is embedded into the high-conduction member without contacting the low-conduction member, thereby to reduce the connection resistance.
However, the portion of the low-conduction member surrounding the pigtail has to be removed to form such a columnar high-conduction cover. This necessitates two steps of pressing and hardening powder material in order to form the brush element. As a result, the manufacturing process becomes complicated and expensive.